1. Field of the Invention
The present invention relates to coating substrates produced by stoving a mixture of polyols, which contain no ester groups, with oxalic acid and to a process for the production thereof.
2. Description of the Prior Art
As is known, polyols may be crosslinked with amino resins or blocked isocyanates at relatively high temperatures (c.f. Ullmann""s Encyclopedia of Industrial Chemistry, Fifth Edition, volume A18, pp. 404-405 and 414-418). Unwanted substances, such as formaldehyde or blocking agents, are eliminated during this operation.
It is an object of the present invention to provide stoving lacquers which primarily eliminate harmless products, such as water.
This object may be achieved in accordance with the present invention by using mixtures of ester group-free polyols and oxalic acid, which crosslink at elevated temperatures to yield solvent-resistant, hard coatings.
The present invention relates to coated substrates obtained by stoving mixtures containing
A) 1 part by weight of ester group-free polyols having an OH content of 1 to 10%,
B) 0.05 to 1 part by weight of oxalic acid and
C) 0.2 to 5 parts by weight of organic solvents, at temperatures of 120xc2x0 C. to 250xc2x0 C. and a stoving time of 1 to 100 min on substrates.
The present invention also relates to a process for preparing the coated substrates.
The stoving lacquer mixtures may be applied onto substrates by knife coating, spraying, flooding or dipping. Stoving then proceeds at temperatures of 120xc2x0 C. to 250xc2x0 C. in the presence of air or inert gas for a period of 1 to 100 min.
Ester group-free polyols A) to be used according to the invention are copolymers prepared from ester group-free vinyl monomers (such as styrene, 3- or 4-methylstyrene, acrylonitrile, alpha-methylstyrene, cyclohexyl vinyl ether, butyl vinyl ether or methyl vinyl ether) with ester group-free, OH-functional, vinyl copolymers (such as allyl alcohol, allyl alcohol hydroxyalkylated with ethylene oxide or propylene oxide, hydroxymethylnorbornene trimethylolpropane monoallyl ether, glycerol monoallyl ether, hydroxyethyl vinyl ether, hydroxybutyl vinyl ether or cyclohexanedimethanol monovinyl ether). Polyols A) are preferably in the form of OH-terminated prepolymers that may be obtained by reacting the preceding polyols with diisocyanates, such as bis(4-isocyanatocyclohexyl)methane, 4,4xe2x80x2-diisocyanatodiphenyl-methane, tolylene diisocyanate, isophorone diisocyanate and hexamethylene diisocyanate.
According to the invention, it is also possible to copolymerize ester group-free monomers with allyl glycidyl ether, such that the epoxy ring is opened by water, monoalcohols or secondary amines and then to optionally form an OH-terminated prepolymer by reacting with diisocyanates.
The reaction products of polyols, such as trimethylolpropane, trimethylolethane or pentaerythritol, or the reaction products thereof with ethylene oxide or propylene oxide, which have been pre-extended with diisocyanates to form OH-terminated prepolymers may also be used according to the invention as component A).
Polyols protected by ketal groups, such as the monoketals of pentaerythritol with acetone, cyclohexanone or benzaldehyde, or the diketals of sugar alcohols, such as sorbitol or mannitol, with acetone, cyclohexanone or benzaldehyde, may also be reacted with diisocyanates to yield polycondensation products, wherein the ketal protective groups are subsequently eliminated.
Preferably, Component A) contains urethane groups.
Component B) is oxalic acid, which, before mixing with the other components, is preferably predissolved in a dipolar-aprotic solvent, preferably N-methylpyrrolidone, dimethylacetamide and/or dimethylformamide, to preferably provide a 10 to 40 wt. % solution.
Solvents C) include aromatic hydrocarbons such as toluene, xylene or mixtures of alkylated aromatics; ester-containing solvents such as butyl acetate, methoxypropyl acetate, methoxyethyl acetate or ethyl acetate; ether-containing solvents such as ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, dioxane or tetrahydrofuran; and dipolar-aprotic solvents such as N-methylpyrrolidone, dimethylacetamide, dimethylformamide, dimethyl sulfoxide, N-methylcaprolactam, dimethyl sulfone or sulfolane. The dipolar-aprotic solvents are preferably used to predissolve the oxalic acid.
Optional components D) include pigments and known lacquer additives.
Components A) to D) may, in principle, be mixed in any desired order at temperatures of below 50xc2x0 C. with shear (e.g. stirring). The oxalic acid is preferably predissolved in a dipolar-aprotic solvent, wherein this solvent may be added to the other components when desired.
The mixture of components A) to D) is stable at room temperature for at least 2 weeks, which means that a storage-stable single component coating composition may be formulated. The solution of oxalic acid in a dipolar-aprotic solvent may, however, also be added immediately before application.
In addition to oxalic acid, it is also possible to add further curing agents such as amino resins, blocked isocyanates or preferably epoxides in quantities of 1 to 20 parts by weight, based on 100 parts by weight of the mixture of A) to D).
The coatings according to the invention may be used on any substrates that are stable at above 160xc2x0 C., such as metals, glass, high temperature plastics or mineral bases. The present invention accordingly also relates to substrates coated with the coating compositions according to the invention.
The coatings are characterised by elevated hardness and solvent resistance. During stoving, only the solvents and water (from esterification of the OH groups with the oxalic acid) are released. No products, such as formaldehyde or isocyanate blocking agents, are released.